Gillis is known for his often existentially-themed plots with complex and very unique storylines. Good examples are his runs on Micronauts: The New Voyages and The Defenders (following runs by Bill Mantlo on the former and J.M. DeMatteis on the latter, both of whom had somewhat similar creative, thoughtful styles). He was noted for focusing on both the humanist aspect of his stories as well as the cosmic aspects.

He also wrote the second volume of Strange Tales, the second volume of The Eternals, and Strikeforce: Morituri. He was the writer for Shatter, an early digitally-drawn comic.